1. Field of the Invention
The present invention is directed to a grounding strap and conductive stretchable fabric adapted to be formed into such strap.
More particularly, the invention relates to a stretchable conductive strap employed for the dissipation of electrostatic charges accumulating on the bodies of individuals.
2. The Prior Art
It is conventional practice for persons working in proximity to explosive environments or sensitive electronic devices to employ means for dissipating electrostatic charges. By way of example, operating room personnel working in proximity to explosive gas mixtures, computer operators, assemblers of electronic chips and the like generate static charges which, if not dissipated, could reach many thousands of volts. If such charges accumulate sufficiently, there is danger of a spark discharge to ground, with possible catastrophic consequences.
Numerous devices have been suggested to effect the desired dissipation, such devices including conductive wrist bands connected to ground.
U.S. Pat. No. 4,398,277 of Aug. 9, 1983, the content of which is herein incorporated by reference, embodies a comprehensive recitation of prior art devices employed for static dissipation and their shortcomings.
The noted patent discloses a knitted stretchable fabric wrist band which includes loops of an elastomeric yarn end plaited with a conductive end. The resultant wrist band is said to be advantagenous in that, by virtue of the stretchability of the fabric, the surface of the fabric having conductive loops is drawn against the wrist of the wearer.
A drawback which inheres in the fabric of the noted patent resides in the fact that, notwithstanding the presence of loops of conductive yarn at one surface of the fabric, there is no assurance that the conductive components will provide an effective electrical connection to the skin of the wearer.
The difficulties inhering in the structure of the noted patent may be better appreciated when it is recognized that in large measure the static discharge straps are employed in controlled environments where temperature and humidity are carefully regulated. In such environments, and especially in environments wherein the humidity is maintained at a low level, effective grounding depends upon direct contact between the crests of the conductive loops and the skin of the wearer.
The problem is most acute in the case of male workers having substantial hair accumulation in the wrist area. In such cases the worker's hair may mat between the band and the skin, in effect providing an insulative sleeve which precludes proper static dissipation.
In the knitted fabric of said U.S. Pat. No. 4,398,277 the loops of conductive yarn tend to shrink in size when the fabric is stretched from the "as knit" condition and lie or be readily deflected to a condition in which they are disposed parallel to the skin of the wearer rather than normal thereto. The fabric structure of such patent lacks means for supporting the crests of the loops against deflection from the desired condition of perpendicularity. With continued use, the tendency of the loops of the fabric to lie in a parallel-to-the-skin, non-hair-penetrating condition increases as the fibers lose resilience. Moreover, as with all knitted fabrics, lengthwise stretching of the fabric results in a widthwise narrowing thereof.
Alternative proposed solutions to the grounding problem appear in U.S. Pat. Nos. 4,373,175 of Feb. 8, 1983; 4,459,633 of July 10, 1984, and 4,475,141 of Oct. 2, 1984. However all such prior art references are deficient in various aspects of comfort of the wearer and efficiency in effecting skin contact.